Abstract: The surface temperature and shell thickness of continuous casts slab were obtained by modeling heat transfer during solidification process. A mathematical model of strain at solid/liquid interface was set up. The strain in the solidifying shell under normal and abnormal operation conditions was gained. The results indicate that strain is small under normal operation conditions and internal cracks never happen. But when the variation of roll gap is above 2 mm, the strain caused is greater than caused by bulging. The total strain exceeds the critical one and internal cracks will appear. It is of great importance to maintain the fine state of continuous casting machine to avoid the appearance of internal cracks.